Original reportWhat's the Relative Risk? A Method to Directly Estimate Risk Ratios in Cohort Studies of Common Outcomes
Section snippets
Selected Abbreviations and Acronyms
ORs = odds ratios
RRs = risk ratios
BMI = body mass index
RD = risk difference
Methods
Recent releases of the SAS System for Windows have included a generalized linear modeling procedure known as PROC GENMOD (10). In PROC GENMOD, the user can specify a number of features of the regression model, such as the distribution of the dependent variable, the link function, and whether an offset term is to be used. To perform the popular logistic regression procedure, users would specify a binomial distribution and a logit link. (If we denote a probability as p, the logit of p refers to
Results
Table 1 shows results from analysis of data from a cohort of 16,778 men on active duty in the US Air Force. These men were given a test that had a binary (pass/fail) outcome, and the values of several predictor variables were measured prior to testing. Only results relating to the effect of obesity, defined as a body mass index (BMI, equal to weight in kg divided by height squared in m2) ≥ 30.0 kg/m2, are shown. The referent group was composed of persons with normal weight, i.e., 18.5 kg/m2 ≤
Discussion
In order to avoid the recurring problems associated with ORs, we advocate that whenever possible, investigators directly estimate and report RRs. Of course, if an outcome is truly uncommon in a study population (< 5%), ORs will give unbiased estimates of RRs. In such cases, the difference between the OR and RR may be so small that some investigators may prefer the more convenient OR. Moreover, in cohort studies, the OR may be used as an index of association on its own, i.e., not as an estimator
Acknowledgements
This work was performed as part of the authors' duties as employees of the US Federal Government, and no other sources of support were involved. The views expressed in this paper are those of the authors only, and should not be interpreted as the official position of the US Air Force or the Department of Defense.
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